HELL CREEK BEDS, CERATOPS BEDS AND EQUIVALENTS 183 



half of North Dakota, eastern Montana as far west as the Bridger 

 Range, western South Dakota, eastern and central Wyoming, and 

 northwestern Colorado. It is a fresh-water formation, consisting 

 of comparatively fine material, mainly clay shale, sands and soft 

 sandstones, with numerous beds of lignite and occasional thin beds 

 of impure limestone. Owing to the many alternations of soft rocks 

 with thin hard layers extensive erosion has given rise to the well- 

 known bad-land topography so characteristic of much of the area 

 covered by the formation in Montana, North Dakota and Wyoming. 

 The maximum thickness of the Fort Union exceeds 8000 feet. It 

 rests, sometimes in apparent conformity and in other cases with 

 unconformity, on various underlying formations. The relationship 

 with lower beds will be discussed later. 



Division of Fort Union Formation into Two Members. 



In many cases — notably in the vicinity of Hell Creek, along the 

 Yellowstone River at Miles City and Glendive, Montana, adjacent 

 southwestern North Dakota, and east of the Bighorn Mountains in 

 Wyoming — it is possible to separate the Fort Union into two mem- 

 bers on the ground of a marked difference in lithologic character. 

 The upper member is composed in the main of light-colored, yellow- 

 ish sandstones and clays, while the lower member is made up of 

 many alternating beds of clay shale and sandstone of a dark gray 

 or somber hue, whence, as a convenient field term, they have often 

 been designated the "somber beds." The contact of the lower and 

 upper members is so sharp, and the contrast in coloration so marked, 

 that they may usually be recognized at a distance and traced with 

 little difficulty. In certain parts of the area a thick bed of coal, or 

 a bed of red baked clay due to the burning out of the coal, marks 

 the point of contact. The present paper deals only with the strati- 

 graphic relations and paleontology of the lower member. 



The dark gray so-called "somber beds" of eastern Montana 

 naturally attracted the attention of those who visited this region. 

 From their stratigraphic position and the sharp lithologic contrast 

 between them and the overlying beds, they were presumed to belong 

 to the Laramie, but recently secured paleobotanical and strati- 

 graphic data shows that they have little or no relation to the Laramie, 



